1. Field of the Technology
The disclosure relates to the field of micromachined inertial sensors and gyroscopes, specifically high range digital angular rate sensors based on frequency modulation.
2. Description of the Prior Art
Vibratory gyroscopes have long be used in the art and maximization of their quality (Q) factors is key to improving performance. Mode matching of conventional high-Q angular rate gyroscopes increases the signal-to-noise ratio at the tradeoff of linear range and measurement bandwidth (10 deg/s range, sub-Hz bandwidth typical for Q˜100 k). These constraints stem from a fundamental Q versus bandwidth tradeoff and dynamic range limitations of analog Amplitude Modulation (AM) systems. In conventional Microelectromechanical Systems (MEMS) gyroscopes, the sense-mode response is excited by the input angular rate amplitude-modulated by the drive-mode velocity.
The operation of conventional vibratory rate gyroscopes is illustrated in the block diagram of FIG. 1B. The drive-mode is a resonator driven to a constant amplitude of vibrations at a fixed frequency by means of a feedback system. Sense-mode vibrations are excited by the Coriolis force which is a product of the drive-mode velocity and the input angular rate. The scale factor is proportional to the sense-mode Q, drive-mode amplitude, and frequency. During rotation of the device, the Coriolis effect causes coupling of energy from the drive-mode to the sense-mode as seen in the graphical representation of FIG. 1A. The equations of motion in the x and y direction for conventional gyroscopes are given in equations 1 and 2 below:
                                          x            ¨                    +                                                    ω                                  n                  x                                                            Q                x                                      ⁢                          x              .                                +                                    ω                              n                x                            2                        ⁢            x                          =                              F            x                    m                                    (        1        )                                                                    y              ¨                        +                                                            ω                                      n                    y                                                                    Q                  y                                            ⁢                              y                .                                      +                                          ω                                  n                  y                                2                            ⁢              y                                =                                    -              2                        ⁢                                                  ⁢                          x              .                        ⁢            Ω                          ⁢                                                      (        2        )            
The Coriolis force applied to the sense-mode is proportional to the input angular rate Ω as well as the drive-mode velocity. Since the drive-mode velocity is a sinusoidal signal with a fixed frequency, the Coriolis effect results in the Amplitude Modulation of the input angular rate by the drive-mode velocity. To measure the input angular rate, displacement (or, equivalently, velocity) of the Coriolis force induced sense-mode vibrations is typically measured. In this conventional architecture, the final output signal of the rate sensors is proportional to the input angular rate, as well as a number of device parameters, including the sense-mode quality factor.
The first fundamental limitation of the described conventional architecture comes from the necessity to precisely measure extremely small analogue signals. In the best case scenario, AM capacitive readout with preselected low-noise electronic components can only achieve a resolution of 1e-6, with a practical limit of 1e-5. This imposes a fundamental limitation on the dynamic range and output stability and precludes MEMS gyroscopes from many potential applications.
The conventional AM based rate sensor operation is also sensitive to the value of the sense-mode Q-factor, resulting in significant response drifts over ambient temperature and pressure variations. FIG. 2 shows an example of sensor sensitivity due to the variation of quality factor over a temperature range using the data published by an independent research group in. The high-Q mode-matched vibratory gyroscope exhibits a 40% drift in its calibration curve for the temperature cycled from 25° C. to 75° C.—a typical performance for a conventional AM based gyroscope. Response of the gyroscope changes by a factor of 2.3 from 25° C. to 100° C., rendering it useless. While temperature compensation using an embedded thermometer is often used to reduce the effect of temperature, the approach limits the accuracy of the sensors and suffers from thermal hysteresis and lag.
What is needed is an angular rate sensor that eliminates the gain-bandwidth tradeoff of conventional AM gyroscopes and enables signal-to-noise ratio improvements through the use of ultrahigh Q structures without limiting the measurement bandwidth. At the same time, the angular rate sensor needs to be robust against mechanical and electromagnetic interferences.